Encouraging results of neutron radiotherapy have been reported from a number of centers throughout the world. Evidence also exists from radiobiology studies that when compared to photons, neutrons produce consistent lethal changes in cells and are less sensitive to effects such as cell cycle and hypoxic conditions. Tissue-equivalent (TE) ionization chambers which are used by radio-therapists to measure dose are not completely TE, and they provide no information concerning the microscopic distribution of absorbed energy. Although standardization of technique, and hence relative values, has been largely achieved, a serious problem in absolute dosimetry remains. The neutron spectra employed at several centers lie largely above 15 MeV, where very little is known about charged particle production cross sections for tissue-resident (TR) elements. These cross sections are required at each neutron energy in the therapy beam in order to correct for the lack of tissue equivalence of the ionization chambers. We have developed the neutron beams and detection systems and have taken data for hydrogen and helium isotope production at 27.4, 39.7, and 60.7 MeV from targets containing C, N, and O. We are reducing, analyzing, and applying the appropriate corrections to these data to obtain charged particle production cross sections. Theoretical calculations indicate that data are needed at more incident neutron energies and more data are needed at some of the energies and angles already measured. We propose (a) the completion of the analysis of data already taken, (b) further measurements with a neutron beam of good energy resolution (approximately 1 MeV) and (c) further theoretical calculations to parameterize the cross sections at all angles and energies. This will allow the calculation of the kerma, dose and LET spectrum for any neutron spectrum so that corrections can be applied to allow high precision dosimetry for the neutron spectra now being used in therapy. In addition a better understanding of dose and LET measurements will be achieved. These calculations are being made by a theoretical group at Naval Research Laboratory.